This invention relates to improvements of a Phase Locked Loop (PLL hereinafter), Frequency Modulation (FM) detection system and more particularly to an IC automatic tuning PLL FM detection system.
A ceramic discriminator or a detection coil is usually used in conventional audio signal detection systems of T.V. and FM radio receivers. PLL FM detection systems employing an external resistor and capacitor are also used.
A complete integration is impossible in FM detection systems using ceramic discriminators and detection coils. In the PLL FM detection system employing an external resistor and capacitor, a complete integration is also impossible because the external resistor and capacitor must be adjusted externally to match the center frequency of the input signal with the center frequency of the lock range.
It is well known that the total harmonic distortion of demodulated signals increases in the detection of high deviation ratio FM signals using ceramic discriminators and detection coils due to their linear properties. In a conventional PLL FM detection system employing an external resistor and capacitor, the loop gain of the PLL system is increased to obtain wide detectable range, which causes a decrease of detection output and deterioration of the S/N ratio due to externl or internal noise.